1. Field of the Invention
The present invention relates to a semiconductor device structure. More particularly, the present invention relates to a semiconductor device structure capable of reducing the hot carrier effect and a method for reducing the hot carrier effect of a metal-oxide-semiconductor (MOS) transistor.
2. Description of the Related Art
MOS transistors are the most important elements in various electronic products. To compactify the electronic products, the dimensions of a MOS transistor have to be reduced. However, such a dimension reduction causes a hot carrier effect affecting the operation of the transistor. More specifically, when the channel length is decreased but the voltage applied is fixed, not only the operation speed of the transistor is increased, but also the lateral electric field in the channel is increased. Thereby, the energy of the channel electrons is increased, while the channel electrons near the drain region can be accelerated to have energy higher than the band gap of the semiconductor.
Therefore, after colliding with the valence-band electrons, the channel electrons near the drain region easily excite the valence-band electrons thereat to the conductive band to form hot electrons. A part of the hot electrons will enter the gate dielectric to cause damages, so that the reliability and the lifetime of the device are reduced.